1. Field of the Invention
This invention relates to an apparatus and process for ultrasonic seam welding of ferrous metal foils, particularly stainless steel foils, that uses a weld wheel or head having a peripheral surface coated with aluminum oxide.
2. Description of the Prior Art
The ability to weld metal foils is an important aspect of quality control in fabricating mechanical components from relatively thin metallic materials. In applications using relatively heavy gauge materials, such as thicknesses of 50 mils and greater, welding processes such as arc welding, brazing and the like are reliable but often costly in terms of labor. It is difficult to produce reliable bonds or to avoid burning out the work materials when welding lighter gauge materials, such as thin strips and metal foils.
In certain applications, such as heat exchangers, spot welds are not adequate for constructing the mechanical equipment. In order to adequately construct a seam weld without using ultrasonic seam welding techniques, it is possible to use either laser welding or electron beam welding techniques. However, such techniques are undesirable since they require quite expensive processes. In addition, electron beam welding is not a reliable process for bonding very thin foils. Ultrasonic seam welding is preferred since it does not melt the weldment or even heat it to annealing temperatures in most cases. The ultrasonic seam welding process operates through cold working. By bonding the materials through a cold working process, corrosion resistance at an ultrasonic seam weld is much higher than corrosion resistance at a seam produced through brazing, or welding, where the weldment is melted and recrystallized.
The ultrasonic welding process can fail when the work material of the foil sticks or bonds directly to the weld wheel or disc, or when two or more metal foils fail to bond together, or when the ultrasonic welding process weakens or damages the metal foils. Although all of these problems have been overcome for many non-ferrous foils, such as aluminum, copper and nickel foils, all three problems occur, either singly or in combination, when attempts are made to ultrasonically weld stainless steel or other ferrous foils using prior art processes or apparatuses.
U.S. Pat. No. 3,217,957 discloses a welding apparatus having a rotatable disc which is acoustically coupled to an ultrasonic vibrator. The disc is adapted for rolling peripheral engagement with the work materials. The ultrasonic frequency vibrations are transmitted to the disc such that the peripheral surface of the disc vibrates at a determined frequency. The disc vibrates in a direction transverse to the direction of its rotation. The '957 patent teaches ultrasonic seam welding of aluminum foils but does not teach or describe either an apparatus or a process for ultrasonically seam welding stainless steel or ferrous foils.
U.S. Pat. No. 4,144,110 discloses a dynamic friction bonding process in which friction is created by high-speed relative movement between a friction tool and a solid surface with which the friction tool is in contact. The '110 patent discloses many different materials that can be cohesively bonded, including dissimilar metals. However, the '110 patent does not specifically mention a process or apparatus for bonding either stainless steel or other ferrous metals through ultrasonic vibration.
U.S. Pat. No. 4,106,167 teaches a dynamic friction bonding process for formation of adhesive or cohesive bonds between metal parts, particularly applicable to the formation of metal cans, drums or other tubular containers. All bonding examples, as disclosed in the '167 patent, relate to tin plated steel, chrome-chrome oxide coated steel, or aluminum. The '167 patent does not disclose an apparatus or process in which bonding occurs primarily because of ultrasonic vibrations or which uses a weld wheel or disc coated with aluminum oxide to prevent bonding between the weld wheel or disc and stainless steel foils.
U.S. Pat. Nos. 3,088,343, 4,333,791, 3,418,196, 4,591,087 and 3,360,850 generally relate to processes or apparatuses for conventional ultrasonic seam welding. The '791 patent discloses an ultrasonic seam welding apparatus which has a cylinder that is vibrated at an ultrasonic frequency; the cylinder is positioned beneath a rotatable weld wheel and the work material is seam welded as it travels between the cylinder and weld wheel.
U.S. Pat. No. 2,833,238 discloses an ultrasonic brazing unit which is used to effect brazed joints with high-melting non-ferrous filler metals. U.S. Pat. No. 3,541,671 discloses a process for fixing paper to a metal foil, particularly an aluminum foil. U.S. Pat. No. 3,899,377 discloses a process for formation of adhesive or cohesive bonding in or to aluminum.
The prior art sufficiently describes processes and apparatuses for bonding non-ferrous metal foils through ultrasonic seam welding. However, the prior art does not disclose a process or apparatus for ultrasonically seam welding ferrous metal foils, particularly stainless steel foils. Thus, there exists a need for a process and apparatus which can be used to ultrasonically seam weld stainless steel foils, without the weld wheel or head sticking or bonding directly to the work material.